Method of molding optical recording drums

ABSTRACT

A method for fabricating in a mold a rotatable recording drum is disclosed. Initially, the mold is rotated and a predetermined quantity of a flowable substance is poured into the mold cavity and is centrifugally flung against the mold wall to form a mold layer with a smooth inner surface. Next, a surface substance is poured into the mold cavity and is allowed to harden against the mold layer. A core outer layer substance is then poured into the mold cavity and allowed to harden against the surface substance, and rotation of the mold is stopped. The mold layer fluid is removed, and the cavity inside the outer layer is filled with a core inner layer substance which is allowed to harden before the drum core is removed from the mold cavity. Following removal of the core from the mold cavity, the hardened surface substance may be vacuum coated, and a protective overcoat applied to the core. The drum core and the surface substance can also be formed separately.

This is a division of application Ser. No. 07/583,668 filed Sep. 17,1990, now U.S. Pat. No. 5,096,627.

TECHNICAL FIELD

The present invention relates to optical recording drums. Moreparticularly, the present invention relates to methods of spin moldingoptical recording drums using solid and liquid mold systems.

BACKGROUND OF THE INVENTION

Rotating drums have been used for the storage of data in electronicdigital computing systems since the first electronic digital computerbuilt in the 1930's. Although magnetic recording superseded these earlydrums, the use of rotatable drum memories has persisted. The generalconfiguration of a rotating drum memory is a cylinder which is rotatedat a constant speed around its axis. A recording medium, such as amagnetic recording material, is deposited on the drum surface. Data isrecorded, usually in bit form, by a recording device located adjacentthe rotating drum surface. The data is fed from a data source such as adigital computer, and is recorded in circumferential lines, called datatracks, on the drum surface. To read the data, a reading device isplaced over the data track to feed the data back to the computing systemas the drum rotates. The time required to find and read a particularitem of data on the rotating drum is the access time.

Recording drums are superior to disks in that the surface velocity of adrum is constant over the entire surface whereas with disks the velocityvaries with the radius. With disks, the maximum data transfer rate is afunction of the innermost ring, and the disk operates at a very lowefficiency when at the outermost rings unless the disk speed varies withthe changing radius. For example, when the outermost rings are twice aslong as the innermost rings, the disk operates at 50% efficiency at theouter rings. Drums operate at a constant velocity and have a higher datatransfer rate as they operate at 100% efficiency. Additionally, becausethe mechanical precision of drums is typically greater than disks, lessstringent performance is required of the optical focusing and trackingservos used with optical drums. While the coating of magnetic media andphotographic emulsions onto drums is technically and economicallyfeasible, the adaptation of the drum configuration to new recordingtechnology such as optical recording is less feasible.

While the first rotating drum memories used capacitors as the recordingmedium, and most commercial drum memories use magnetic media, opticalmemory rotating drums have been disclosed in U.S. Pat. Nos. 3,383,662;3,408,634; 3,440,119; and 3,500,343. In the optical drum memorydisclosed in this group of patents, the outside of a cylinder is coatedwith a photographic emulsion, and data bits are recorded on theresulting photosensitive surface. Data is read from the cylinder by amicroscope and a photodiode. This device provides greater bit densitythan prior magnetic recording media, and is insensitive to strongmagnetic fields. However, this device can only record dataphotographically; the data must be developed and can not be rewritten.Thus, this device can not interact with associated computers or otherdigital systems in real time as it is read only.

In optical recording technologies using laser recording, a laser beam isfocused to a very small spot to record data onto an optically sensitivecoating on a substrate. The substrate is an inert substance on which theoptically sensitive layer is coated. The data is immediately readableafter recording, without any intermediate processing such as chemicaldevelopment of latent images. Such recording systems are called directread after write (DRAW) systems. Such a recording mode is permanent andtherefore is not erasable and reusable.

Magneto-optic recording is an erasable, reusable method of laserrecording, and uses a tightly focused laser spot to heat an area ofmagnetic material above its Curie point while subjecting the area to amagnetic field. The size of the recorded data bit is determined by thesize of the heated area, and is smaller than the area covered by themagnetic field. Therefore, bit areas much smaller than those achievablewith conventional magnetic recording heads can be obtained. Themagnetically recorded bits can be read by a laser beam.

Magneto-optic recording can attain bit densities as great as ten timesthat of rigid disk magnetic media as the data bits have an area on theorder of one square micrometer, and the bit and track pitch are ofsimilarly small dimensions. To achieve these levels of resolution, thesurface of the recording medium must be extremely smooth, a conditionnot easily producible in cylindrical form with existing technologies.Moreover, apparatus for locating the data on such a small scale must beextremely precise.

Focusing a beam of light, usually a laser beam, to a sufficiently smallspot size to achieve resolution on the order of one micrometer requiresthe distance from the focusing lens to the recording surface to be heldto tolerances of one micrometer. While reading a data track, therecording medium surface inevitably moves in a direction normal to itsaxis of rotation, thereby changing the lens-to-surface distance. Withrotating drums, this surface wandering is expressed as the totaldistance the surface wanders during one revolution of the drum and iscommonly referred to as runout, or total indicated runout (TIR). Tocompensate for this surface wandering, servo systems maintain focus byadjusting the lens location as the surface wanders. In optical diskrecording, surface wandering can be over 100 micrometers, and servosystems can adjust the lens location to a tolerance of less than 1micrometer at typical rotational speeds. As surface wandering inrotating drum memories is considerably less, a simpler focusingmechanism can be used, while providing greater focus accuracy and higherspeeds.

Typical diameters of commercially available rotating drum memories arein the range of 26.7-83.3 cm (10.5-32.8 inches). Maintaining surfacewandering to within a few micrometers for these drums requires greaterprecision than is achievable by conventional manufacturing processes,absent expensive and time-consuming finishing operations. Surfacewandering of cylindrical drums is caused by the bearings, theeccentricity of the drum surface, and various surface waves,out-of-roundness, and other defects. Even assuming a stationary drumcenter, the large drum size contributes to dimensional variations whichwould lead to surface wandering. Furthermore, the large mass and anyvibration-causing unbalance increase surface wandering.

Finally, manufacture is complicated by the high level of smoothnessrequired on the outside of the drum. While techniques for producingsmooth surfaces on flat optical recording disks are known, thesetechniques are not suitable for applying similar surfaces to drums. Thesmooth flat recording surfaces on optical disk recording media can beachieved by coating a curable polymeric liquid onto a horizontalsubstrate to form a free surface and allowing the liquid to harden. Thecoating can be thinned by spinning the horizontal substrate around avertical axis, before hardening, thereby flinging off excess material bycentrifugal force. This coating process, referred to as spin-coating,provides very high quality flat surfaces, but can not coat the outsideof a drum, as cylindrical surfaces cannot be made horizontal, andgravity causes sagging and other non-uniformities.

Nonetheless, spinning a liquid layer around a vertical axis can formsymmetric curved optical surfaces in a technique known as spin casting.Various aspects of spin casting contact lenses are described in U.S.Pat. Nos. 4,416,837; 4,534,915; 4,637,791 and 4,659,522. In the firstpatent, a mold is spun and used to spin cast contact lenses. In thesecond patent, UV light is used to minimize stresses by curing the castproduct more rapidly near the center. In the third patent, vibration isreduced during spin casting to prevent surface waves in the lens duringcuring. The last patent describes spin casting of an annular lens. Thesepatents illustrate that spin casting can form optical quality concavesurfaces, and that a spin cast polymeric mold can, in some cases, beused to produce an optical quality convex surface.

However, the lenses produced by spin casting in these patents bearlittle relation to the optical quality surfaces required in a rotatingdrum used in laser optical recording. Contact lenses are not rightcircular cylinders and do not require precise mechanical tolerances ofthe type required of memory drum components. Moreover, when placed inthe eye, contact lenses are covered by liquid layers which coat surfaceroughness. Optical recording involves no liquid layer and requires ahigh level of smoothness to prevent the optically sensed signal frombeing lost in noise generated by roughness.

SUMMARY OF THE INVENTION

The present invention provides an optically smooth, concentriccylindrical surface by providing a layer on the surface of a cylindricaldrum which exhibits levels of smoothness normally achieved only bycoatings or by complex grinding and polishing procedures. Thecylindrical surface eliminates larger scale eccentricities,out-of-roundness, and other sources of surface wandering found in drumsproduced by conventional volume production methods.

A method for fabricating a rotatable recording drum in a singlecylindrical mold rotatable around an axially central mold shaft includesthe following steps. First, the mold is rotated around its axis ofrotation and a measured, predetermined quantity of a flowable substanceis poured into the mold cavity. This substance is centrifugally flungagainst the mold wall to form a cylindrical mold layer having aninterior surface equidistant from the axis of rotation to create aconcentric, smooth, circular inner surface. Next, a surface substancehaving a lower density than the mold layer, such as activated epoxy, ispoured into the mold cavity and is allowed to harden against the moldlayer.

A drum core first layer substance such as a mixture of glassmicrospheres and epoxy is then poured into the mold cavity and allowedto harden against the epoxy, and the mold rotation is stopped. The moldlayer fluid is removed from the mold cavity, and a metallic sleeve isplaced over the mold shaft to serve as a central hole for the drum core.The cavity between the drum core first layer and the metallic sleeve isfilled with a drum core second layer substance such as a paste ofceramic spheres and activated epoxy which is also allowed to harden.Finally, the drum core is removed from the mold cavity.

Where the mold axis of rotation is vertical, the axis is slowly shiftedwhile the mold is rotating while containing both the flowable substanceand the activated epoxy. The mold is shifted to a horizontal position toremove the wedge shape of the profile between the flowable substance andthe epoxy which is caused by the gravitational force. Alternatively, theaxis of rotation can remain at a constant 30° angle with the horizontal.

Following removal of the drum core from the mold cavity, the activatedepoxy hardened on the drum core may be vacuum coated with amagneto-optic, thin film, metallic layer identical in composition to thelayers in magneto-optic recording disks. The drum core is replaced intothe mold cavity, and a protective overcoat is applied to the surface ofthe magneto-optic thin film to protect it and to prevent oxidation ofthe metals in the thin films. This protective overcoat is applied byrotating the mold around its axis of rotation; pouring a measured,quantity of the flowable substance into the mold cavity to form a moldlayer; centrifugally flinging the flowable substance so that itsinterior surface is equidistant from the axis of rotation to create aconcentric, smooth, round inner surface spaced from the outer surface ofthe drum core; inserting the protective overcoat material into the moldcavity between the outer surface of the epoxy and the inner surface ofthe mold layer to fill the space therebetween; and allowing theprotective overcoat to harden onto the outer surface of the drum core.

Alternatively, the drum core and the surface substance can be formedseparately. In forming the core, the mold is first rotated around itsaxis of rotation and a measured, quantity of a flowable substance ispoured into the mold cavity. The flowable substance is centrifugallyflung against the mold wall to form a cylindrical mold layer having aconcentric, smooth, round inner surface. A drum core first layersubstance is poured into the mold cavity and allowed to harden beforethe mold rotation is stopped. The mold layer is removed from the moldcavity, a metallic sleeve is placed over the mold shaft to serve as acentral hole for the drum core, and the cavity between the drum corefirst layer and the metallic sleeve is packed with a drum core secondlayer substance. The drum core second layer substance hardens, and thedrum core is removed from the mold cavity.

To form the surface substance on the preformed core, the preformedcylindrical drum core is placed in a cylindrical mold which may be thesame or different from the mold used to form the core. This mold isrotated around an axially central mold shaft, and a measured, quantityof the flowable substance is poured into the mold cavity to form a moldlayer as discussed above. A surface substance is inserted into the moldcavity between the outer surface of the drum core and the inner surfaceof the mold layer to fill the space therebetween and is allowed toharden onto the outer surface of the drum core. Finally, rotation of themold is stopped and the drum core is removed from the mold cavity.

These methods form a rotatable recording drum for use in erasableoptical recording having an outer surface substance which is anactivated epoxy and an inner core layer formed of microspheres within anepoxy. The drum may also include an outer core layer disposed betweenthe inner core layer and the outer surface. The outer core layer is amixture of epoxy and ceramic microspheres. Alternatively, the inner corelayer could be a mixture of epoxy and glass or ceramic microspheres.

The liquid mold system has several advantages over a solid mold system.First, the finished part will not stick to the mold surface, and themold need not have any taper to facilitate removal of the finished part.Second, the mold is not damaged by scratching or adhesion duringcasting. Third, the mold shell can be used to apply more than one layerto the drum section without having to remake the mold, since the innerdiameter of the layer can be varied by changing the amount of liquidadded to the mold shell in each casting operation. Finally, in liquidmolding systems the smoothness and cleanliness of the cast surfaces aredetermined almost entirely by the purity of the mold liquid, rather thanby the cleanliness of a solid mold surface. Methods of purifying liquidsare simpler than comparably effective methods of cleaning solidsurfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a single drum section. FIG. 1B is aperspective view of several drum sections assembled onto a shaft to forma complete drum. FIGS. 1C and 1D are cross-sectional views of coreconstructions of the drum sections.

FIG. 2 is a cross-sectional view of a mold shell apparatus.

FIGS. 3A-3C are cross-sectional views showing the steps taken infabricating a drum section without a prefabricated core. FIG. 3A showsthe mold shell spinning after the liquid mold layer and surface layerhave been deposited. In FIG. 3B, the outer core layer has beendeposited. In FIG. 3C, the mold shell rotation has been stopped, and thecenter sleeve and inner core are in place.

FIG. 4 is a cross-sectional view of the mold shell with air bladders.

FIG. 5 is a cross-sectional view of the liquid mold system, withprefabricated core, the liquid mold layer and the cast surface layer.

FIGS. 6A and 6B are cross-sectional views of the mold shell duringoperation before and after reorienting the axis of rotation.

FIG. 7 is a cross-sectional view of a solid mold system with aprefabricated core in place, just prior to beginning the injection ofthe casting resin to form a surface layer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The apparatus and method of the present invention produce cylindricalsurfaces which exhibit low runout during rotation, and which producesufficiently smooth surfaces suitable as optical recording substrates.The surfaces produced are located on the curved peripheral surfaces 10of drum sections 12 shown in FIG. 1A. The drum sections 12 can bemounted side-by-side coaxially on a shaft 14 to form a drum 16 usable asa rotating drum memory, as shown in FIG. 1B. As shown in FIG. 1C, thedrum sections 12 include a core 18 and a substrate or surface layer 20.Additionally, the core 18 may include two or more layers, as shown inFIG. 1D in which the core 18 includes a central sleeve 22, an inner core24, and an outer core 26. Fabrication of cores 18 from non-metallicmaterials reduces cost, weight, and the susceptibility of the core 18 tomagnetic fields.

Drum sections 12 may have protrusions or indentations (not shown) on theflat side faces 28 which mate with corresponding parts on adjacent drumsections 12 to link together and facilitate cementing drum sections 12together into a drum 16 during use. Alternatively, linking the drumsections 12 may be accomplished by adhesive. Using separate drumsections 12 simplifies fabrication, and permits adding or removing drumsections 12 on a shaft, to change the capacity of the rotating drummemory.

MOLD SHELL APPARATUS

The methods of fabrication of drum sections 12 use the mold shellapparatus 30 shown in FIG. 2, which can fabricate drum cores 18 andcomplete drum sections 12 using various different materials. The basicapparatus 30 includes a cylindrical mold having an outer mold shell 32rigidly and coaxially mounted to a rotatable shaft 34. The mold shell 32may be machined from a dimensionally stable material such as aluminum.Because the mold shell 32 is spun at rotational velocities above 3000RPM, the density of the mold shell material should be low, to reduceweight and minimize the effects of any residual unbalance. The moldshell 32 also should be sufficiently strong to resist deformation by thecentrifugal forces arising from spinning.

In some cases, such as when photocurable resins are cast, part or all ofthe mold shell 32 may be made from materials transparent to light, suchas ultraviolet light, which is used for curing. Polymeric materials areparticularly suitable for their transparency and ease of fabrication.Additionally, a variety of plastic tooling compounds, particularlyepoxies, can be used to cast part or all of the mold shell 32. Polymericmaterials used for the mold shell 32 must be dimensionally stable, toprevent dimensional changes during use because of warping, heatrelaxation, or other uncontrolled phenomena.

The portion of the shaft 34 which lies within the cavity 36 of the moldshell 32, the critical dimension region, must exhibit a very low levelof runout when the shaft 34 rotates and should closely approximate aperfect cylinder. This critical dimension region of the shaft 34 shouldlocate the geometrical center of produced drum sections 12 within 12.7micrometers (0.0005 inches) of the rotational axis of the rotating drummemory shaft used with the drum section, and the shaft 34 diametershould correspond to the diameter of the rotating drum memory shaft. Theshaft 34 must be large enough to resist bending during use, while smallenough to reduce weight and expense, and improve dimensional accuracy.Shaft 34 may be hollow.

The rotatable shaft 34 is supported on bearings 38 to fix its axis ofrotation. Gas lubricated bearings, such as air bearings which provideless than 0.051 micrometers (2 microinches) of runout, are preferredwhen the apparatus is well balanced. Non-rotational movement could occurin journal bearings if bearing clearances are too large, or in ball orroller bearings if the bearings' centers of rotation are not coincidentwith the center of the shaft 34. Additionally, ball and roller bearingsare less desirable because they produce high frequency vibrations.

The upper edge of the mold shell 32 is covered by a removable annularretaining ring 40. The retaining ring 40 provides an inner lip over thetop of the mold shell 32 to prevent liquid from spinning out of theshell 32, while leaving open the majority of the top of the mold shell32. This is accomplished because the inner diameter of the retainingring 40 is always smaller than the inner diameter of the liquid in themold shell 32. The liquid is always poured into the mold shell 32 whilethe mold shell 32 is spinning so centrifugal force holds the liquidagainst the outer wall of the mold and below the level of the retainingring 40. The retaining ring 40 may be made of aluminum or stainlesssteel, or a chemically stable, non-adhesive polymeric material such aspolytetrafluoroethylene and can be attached to the mold shell 32 bymachine screws.

An upper gasket 42 is placed between the retaining ring 40 and the moldshell 32 to form a liquid-tight seal. The upper gasket 42 may be a softmaterial, such as fluoroelastomers or silicone rubber, which ischemically unreactive with and does not adhere to the casting compounds.The upper gasket 42 may be flat, as shown in FIG. 2, or it may be anO-ring residing in an annular groove cut in the top edge of the moldshell 32. The upper gasket 42 may alternatively be a thin coating ofsealant applied to the upper edge of the mold shell 32, to the bottomsurface of the retaining ring 40, or to both.

Where a preformed inner core 18 is used to make the drum section 12, asshown in FIG. 5, a lower gasket 44 may be placed in the bottom of themold shell 32. The lower gasket 44 may be formed from an elastomericmaterial such as fluoroelastomers or silicone rubber. Alternatively, thelower gasket 44 may be an adhesive sealant applied as a thin layer tothe bottom of the mold shell 32 capable of releasing any finished partsto be removed from the mold. Beeswax, which can be melted onto the moldshell bottom, and releases when melted at temperatures from 61° C. to67° C., can be used. A layer of polytetrafluoroethylene also can beused.

A variable speed electric motor 46 is coupled to the shaft 34 throughone or more belts 48. The motor 46 is mounted to a support base 50through rubber mounts and O-ring belts (not shown) and air bearings 38to prevent the transmission of vibrations to other components.Alternatively, the motor shaft 46A can be connected directly to the endof the air bearing shaft 38A of the air bearings 38.

The orientation of the axis of shaft 34 of the mold shell apparatus 30can be varied, as discussed below, via a gimbal mount 52 shown in FIG.2. The mold shell can be held in the desired position by mechanicalstops, locking worm gears, or pinion mechanisms (not shown).

Once the mold shell apparatus 30 has been balanced, it can be used toproduce a cylindrical mold which is coaxial with the axis of rotation ofthe shaft 34 and which has virtually smooth, perfect interior wallsurfaces. The mold shell 32 is rotated by the motor 46 at a speed whichproduces a centrifugal force at the inner surface of the mold shell 32of about 1800 times the force of gravity, 1800 g. The speed required toachieve this force depends upon the inner radius r of the mold shell 32,and can be derived from the well known relationship

    Z=rω.sup.2 /g                                        (1)

where Z is the ratio of centrifugal force to gravitational force, ω isthe angular velocity in radians per second, and g is the accelerationdue to the earth's gravity. Thus, in revolutions per minute (RPM),

    ω(RPM)=9.549[gZ/r].sup.1/2                           (2)

When the shaft is rotated at speeds such that Z is above 1000, a smallamount of liquid placed in the mold shell 32, with the shaft 34 orientedvertically will form a substantially vertical thin layer lining theinner wall of the mold shell 32. As the centrifugal force is hundreds oftimes greater than the gravitational force, the thin liquid layer formsa smooth surface, which is critical to the operation of the mold shellapparatus 30. However, the gravitational force causes the inner freesurface of the layer to deviate from vertical and be perpendicular tothe resultant of the centrifugal and gravitational force vectors. Theshape of this free surface is a paraboloid of revolution with an axiscoincident with the axis of rotation of shaft 34 and has the equation:

    y=ω.sup.2 x.sup.2 /2g                                (3)

where y is measured along the axis of rotation, and x is the location ofthe point under consideration. Solving equation 3 for x and taking thederivative, and noting that because the layer is quite thin incomparison with radius of the mold shell 32, x can be approximated by r,yields:

    dx/dy=g/[ω.sup.2 r]                                  (4)

where dx/dy is the rate at which the radius of the inner surface of thethin layer changes along the axial direction of the mold shell 32. Therate of change in diameter of the inner surface of the thin layer,commonly referred to as the taper, is 2dx/dy, and is inverselyproportional to the square of angular velocity ω. The taper, as well asthe high degree of smoothness and concentricity of the free surface ofthe thin liquid layer, are the features most desired in molds suitablefor use in making drum sections for optical drum memories.

The mold shell apparatus 30 described above can be used with variousnovel methods to form optical recording drums. In the preferredembodiment, the drums are formed using a single liquid mold as describedbelow. Alternatively, the core 18 and the surface layer 20 can be formedseparately, and a solid mold system also can be used.

THE DRUM CORE MATERIALS

The drum can include a core and an outer surface on which reading andwriting can be performed. The core is preferably formed of inner andouter layers but can simply be a single layer. The core 18 can beconventionally machined from a flat plate of aluminum or other suitablylightweight, dimensionally stable, durable yet workable metal. Thecentral hole must be precisely machined to achieve low levels of surfacewandering when the drum section 12 is placed in a rotating drum memory.However, metal cores 18 are very heavy, expensive to fabricate, mayrequire special balancing operations, and are electrically conductiveand subject to the magnetic field effects present in many recordingprocesses. Therefore, dimensionally stable polymeric resins or othernonmetallic materials or composites are preferred. Non-metallic coresare lighter, less prone to thermal expansion, less affected by themagnetic fields which are often a part of the recording process, andless expensive to fabricate. As nonmetallic resin cores 18 may lackdimensional stability if made with known methods, unique methods offabricating mixtures for nonmetallic cores which meet the requirementsfor rotating drum memories are described below.

Small hollow glass or ceramic spheres can be used with nonmetallicresins to enhance dimensional stability, improve strength, and reduceweight. Spherical fillers can be added to the resin in larger amountswithout unduly increasing the viscosity of the composition. Sphericalfillers are available in various sizes and compositions. Spheres havingdiameters of 200 micrometers or less, known as microspheres, areparticularly useful when surface quality is an important consideration,since their small size permits a fine surface texture. Where surfacequality is less important, larger macrospheres, which can be a fewmillimeters in diameter, are easier to handle and lower in cost.

In a composite core using both microspherical and macrospherical fillersas shown in FIG. 1D, the inner core 24 includes macrospherical fillers,and the outer core 26 includes microspherical fillers. This improves thequality of the surface to which the surface layer 20 adheres. Because ofthe different densities of the layers 24 and 26, their interface shouldbe concentric to the center of rotation of the finished drum section toprevent unbalance of the core.

Suitable resins should have a sufficiently low viscosity to flow intothe narrow gap between the core 18 and a mold layer 54, and cure at ornear room temperature without producing gas bubbles or othercontaminants. Also, the cured resin should adhere to the core 18 but notto the metallized mold surface, and should not shrink excessively duringcure. It is desirable that the casting resin be of microelectronicgrade. Finally, the casting resin, when cured, should be capable offorming a substrate suitable for the optical recording medium to be usedfor the rotating drum memory.

LIQUID MOLD SYSTEM WITHOUT PREFABRICATED CORE

Fabrication of drum sections is preferably accomplished by casting thedrum section 12 in layers, starting from the surface layer 20 andworking inwardly, as illustrated in FIG. 3. This is done using a liquidmold system in which a liquid mold layer 54 is formed in the mold shell32 to serve as the mold surface. Liquid mold systems are better thanmolding within a machined surface because the molded material cannotadhere to the liquid surface, and because the molded surface replicatesthe smooth inner surface of the liquid.

The empty mold shell 32 and the inside of the pouring lip, retainingring 40, can be coated with a light coating of beeswax to seal theretaining ring 40 to the mold shell 32 and to serve as a thermallyexcited coating which releases the finished drum when the mold shell 32is heated.

The first step in forming a drum section 12 is to place the mold shellapparatus 30, with retaining ring 40 in place, in a vibration freeenvironment. Next, the empty mold shell 32 is rotated around a verticalaxis. When the required rotation speed is reached, a measured quantityof mold liquid is deposited into the mold shell 32, where it forms moldlayer 54 against the inner surface of the mold shell 32. The axis of theshaft 34 may be horizontal to eliminate any wedge shape of the freesurface of the mold layer 54 due to the effects of gravity and to form amore optically perfect drum section 12.

The mold liquid forming the mold layer 54 must be of a densitysignificantly higher than the casting resin used to form the surfacelayer 20, and should be chemically inert with respect to, and immisciblewith, the material forming the surface layer 20. A suitable liquid is afluorocarbon of the type commonly used in many electronic manufacturingprocesses, which is available in specific gravities over 1.8. Thepreferred material is a fluorocarbon, Type FC-5311, manufactured by ISCChemicals, Limited, U.K. It has a specific gravity of 2.08. The chemicaldescription of the material is phenanthrene,tetracosafluorotetradecahydro-(cas 306-91-2). Since many castingmaterials suitable for the surface layer 20 have specific gravitiesbelow 1.1, they will float on the inner surface of the fluorocarbonfluid under the effects of centrifugal force and will float on top underthe influence of gravity. Also, fluorocarbon liquids are chemicallystable and do not interfere with curing or react with the castingmaterials. Additionally, due to their chemical stability, fluorocarbonliquids are reusable after filtration, using filters having submicronpore sizes, or other purification. For example, centrifugationeliminates light impurities which migrate to the interface between thelayer 54 and the surface layer 20 during casting.

The mold liquid should not contain contaminants which are of lowerdensity than the liquid itself, since the centrifugal force of thespinning mold shell 32 would cause these particles to move to the innersurface of the mold layer 54 and produce defects in the surface of thesurface layer 20. Furthermore, the mold liquid should not containdissolved impurities which would leave an undesirable film on the castsurface. As fluorocarbon liquids are poor solvents, it is unlikely thatthey will dissolve any materials which might later be deposited as filmswhen the fluorocarbon evaporates.

Once the mold layer 54 is formed in the mold shell 32, a layer of liquidcasting material, such as liquid casting resin, such as an activatedepoxy, is deposited onto the free surface of the layer 54 to form thesurface layer 20 having a thickness of about 2.5 mm (0.1 inch). Therotational speed of the mold shell 32 is maintained until the castingresin cures and hardens. This surface layer 20 forms the outside surfaceof the drum section 12. Preferably, the resin is placed in a vacuumprior to pouring to extract any air that was entrapped during mixing aslarge bubbles could affect the surface profile of the resin.

Materials suitable for use as casting resins include acrylics,polyesters, and epoxies. The primary requirements for the casting resinsare that they cure at or near room temperature, that they do not shrinkexcessively or undergo dimensional changes during curing, and that theyform optically smooth, durable surfaces suitable for optical ormagneto-optic recording materials. Moreover, it is highly desirable thatthe cured casting resin adhere well to the core 18. Cure times could bereduced by using a resin system curable by ultraviolet light. This wouldrequire that the mold shell 32 and the layer 54 be transparent toultraviolet light.

Because the surface layer 20 is thin, delicate, and easily deformed, asecond support layer or outer core 26, shown in FIG. 3B, is applied ontothe inner surface of the hardened surface layer 20, without stoppingrotation. The material used for layer 26 is chosen primarily forstrength and dimensional stability, rather than surface quality oroptical properties. A suitable material is a mixture of glass or ceramicmicrospheres and epoxy having sufficiently low viscosity. This materialis poured into the spinning mold and is spread out by centrifugal forceto form a uniform layer 26 covering the surface layer 20. A sufficientamount of epoxy/microsphere mixture is added to give layer 26 athickness of about 12 mm (0.5 inches).

Once the outer core layer 26 has cured, rotation of the mold shell 32 isstopped, and the liquid layer 54 and the retaining ring 40 are removed.A solid, preferably metal, center sleeve 22 is then placed over theshaft 34, as shown in FIG. 3C. The sleeve 22 provides a precise anddurable inner diameter for the drum section 12. Therefore, the innerdiameter of the sleeve 22 must precisely fit the shaft 34, withsubstantially zero clearance, but with no interference. The sleeve 22should be sufficiently thick to provide adequate strength anddimensional stability both during fabrication and during use of thefinished drum 16. The length of the sleeve 22 should be equal to thewidth of the drum section 12. Since the core 24 should adhere well tothe sleeve 22, the outer surface of the sleeve 22 may be toughened,primed, or otherwise treated to promote adhesion. In an alternativeembodiment, no sleeve 22 is used and the finished drum section 12 iscontacted at its axial ends for rotation rather than being placed on ashaft 14.

The final step in casting the drum section 12 is to form the inner core24. Suitable compositions for forming the core layers 24 and 26 includea binder resin, such as an epoxy and hardener blend, and a hollowspherical filler particulate. The core 24 preferably includes a pasteformed of a mixture of activated epoxy and hollow glass or ceramicspheres. Ceramic spheres are preferred to glass spheres as the ceramicspheres are comparatively inexpensive, lightweight, and strong. Asuitable epoxy blend can be prepared by combining 35 parts of a modifiedcycloaliphatic amine hardener with 100 parts by weight of a lowviscosity, non-crystallizing bisphenol A epoxy. A suitablemacrosphere-filled inner core layer 24 can be prepared by adding 600milliliters of the epoxy blend to 4000 milliliters of ceramic sphereshaving diameters in the range of about 0.6-1.4 mm and stirring until auniform dispersion is obtained. A coarser grade of ceramic sphereshaving particle diameters in the range of 1.4-2.8 mm results in acoarser surface texture. Despite the high proportion of filler inrelation to the amount of epoxy resin used, the resin readily wets thefiller particles and the resulting mixture exhibits flow propertieswhich are very satisfactory for casting purposes.

Although the same binder resin can be used for both layers 24 and 26,the outer core layer 26 requires a smoother, finer textured surface overwhich the final surface is placed. Thus, microspherical fillers, ratherthan macrospherical fillers are used for layer 26. A suitablecomposition can be prepared by preparing a batch of the epoxy blenddiscussed above, and adding hollow glass microspheres until theviscosity approaches the maximum suitable for casting. It is importantthat the filled epoxy blend spreads evenly when deposited in thespinning mold.

The layer 24 is formed by filling the mold cavity between the sleeve 22and layer 26 with the paste, leveling the top surface, and removing theexcess material. As the paste cures, it bonds to the sleeve 22 and formsthe body of the drum section 12. The finished drum section 12 can thenbe removed from the mold by overturning the mold shell 32, and allowingit to slide down the shaft 34. Where beeswax is used, the mold shell 32is heated to melt the beeswax to remove the drum section 12. Any excesscasting material can be removed from the drum section 12 by conventionalmachining, while preventing damage to or contamination of the curvedsurface 10 of the drum section 12. Additionally, excess epoxy connectsthe drum section 12 to the retaining ring 40 to help remove the sectionfrom the mold shell 32. To further assist removal, a tool withcompressible O-rings (not shown) can be used to create an interferencefit with the drum section 12 and pull it out of the mold shell 32.

The resulting drum section 12 can be cleaned by standard cleaningmethods. As the fluorocarbons evaporate without leaving significantresidue, special cleaning procedures are not necessary. Next, an opticalrecording material, such as a magneto-optic medium, can be applied byknown vapor deposition methods such as vacuum coating. Typical materialsinclude those suitable for magneto-optic recording, such as combinationsof cobalt, chromium, rare earth metals, or other magneto-optic thin filmmetallic layers used in magneto-optic recording disks. Alternativerecording materials include those capable of phase change duringrecording.

A protective layer can be provided over the recording medium layer toprotect it from mechanical damage and corrosion, and reduce thedisruptive effects of dust or other contaminants. Thicker overcoatlayers, called dust defocusing layers, reduce optical reading noise dueto dust since the light beam used for reading has a very small depth offocus. Therefore, any dust on the overcoat surface will be outside thefocal plane of the beam, and will contribute less to the noise level.Materials useful as overcoats are optically transparent and mechanicallydurable. However, overcoating materials which are cured by UV light areusable if the mold shell 32 and the layer 54 are transparent to therequired wavelengths. "Hard coat" plastics, such as the hard materialscoated over eyeglasses, are desirable and would be hardened by exposureto UV light through the mold shell 32 and layer 54.

Where recording is magneto-optic, the overcoat preferably is notbirefringent to the plane polarized light used to read the data.Overcoating resins which do not develop large strains upon curing, asmight occur in casting resins which exhibit high shrink rates, are lesslikely to produce birefringence.

The overcoat can be applied with the molding apparatus used to apply therecording substrate to the drum section. After the optical recordingmaterial has been applied, the drum section 12 is placed in the moldshell 32, and the retaining ring 40 is attached. Rotation is started asbefore, and when a suitable, reduced speed (e.g., 450 rpm) is reached, ameasured quantity of mold liquid which forms layer 54 is deposited ontothe top of the rotating core 18. The mold liquid is flung outwardly,caught by the ring 40, and directed downwardly along the inner wall ofthe mold shell 32. The quantity of mold liquid used is less than thatused for casting the surface layer 20, as the inner surface of the layer54 is farther from the core 18, due to the thickness of the surfacelayer 20 already applied.

Once the mold liquid is in place, the overcoat layer is applied. Again,the rotational speed is increased, and the mold is slowly rotated intothe horizontal position as curing occurs. Finally, the drum section 12is removed from the mold, and any excess edge material is removed. Thisis a particularly convenient point at which to perform final machiningor other finishing operations, since the dust defocusing layer willprotect the recording surface from any contaminants resulting from suchoperations. Once the final finishing is done, the drum section 12 isready to be mounted on a drum shaft, perhaps with other drum sections,to form a rotating drum memory.

FABRICATION OF CORES

Alternatively, the drum core 18 can be preformed, and the surface layer20 formed around the core 18 in a mold apparatus 30. This allows thecore-forming and surface-forming operations to be performed in parallel.Depending upon the material used for the core 18 and the intermediatelayers, several drum section configurations can be produced.

In forming the core 18, after the retaining ring 40 is in place, themold shell 32 is rotated at the required speed, and a measured quantityof the mold liquid is deposited into the spinning mold shell 32 to forma mold layer 54. Next, the outer core 26 is formed by depositing aquantity of the casting epoxy resin containing hollow microspheres intothe rotating mold. As the density of the microspheres are less than theepoxy resin, the outer surface of this outer core layer is higher inepoxy content, and is smoother than the inner portion of the layer. Oncethe epoxy/microsphere layer has cured, rotation is stopped, theretaining ring 40 is removed, the center sleeve 22 is positioned onshaft 34, and the inner core 24 can be cast.

The inner core 24 is formed by filling the remaining volume of the moldwith the epoxy and hollow ceramic macrosphere mixture described above.The top surface is leveled and smoothed, and the inner core 24 isallowed to cure. The core 18 is removed from the mold, and finishingoperations are performed.

In order to keep the weight, inertia, and cost of prefabricated cores 18low, the core material must combine light weight with strength and itmust be low cost. Ceramic spheres, such as those manufactured in 1990 by3M under the trade name Macrolite™, meet these requirements. When wettedwith epoxy resin and allowed to cure in a solid state, the ceramicspheres produce a lightweight solid that is hard and has high tensilestrength to resist high centrifugal forces encountered in rapidlyspinning drum applications. This material is low in cost.

Alternatively, prefabricated drum cores 18 can be formed as a singlelayer by pressing the paste-like substance, produced by wetting theceramic spheres with epoxy, into a cylindrical cavity without usingcentrifugal force. The cavity can be a mold shell 32, having dimensionsidentical to the desired size of the finished core. A metallic sleeve 22is slipped over the shaft 34 prior to inserting the ceramic spheres, andthe sleeve 22 becomes the central structural member of the drum section12 used for mounting the core 18 and for locating the core 18 duringsubsequent drum section 12 fabrication processes and in final mountingin optical recording systems. The epoxy which wets the ceramic spheresalso cements the metal sleeve 22 to the ceramic sphere core. This simpleprocess, however, produces drum sections 12 which are very porous at theperimeter, and which therefore provide poor surfaces on which to coatoptical layers by the spinning processes described above. Also, voidscan be created in the interior mass of the cores 18 which can cause thedrum sections 12 to be unbalanced when rotated in subsequent fabricationprocesses and in use as optical drums.

Both of these problems are eliminated by using a core mold which is spunat several hundred rpm for a short time after the wetted ceramic spherematerial is placed inside the mold, and which is then rotated atapproximately 30 rpm until the epoxy hardens. This mold differs fromthose described above in that rather than using a retaining ring 40, arigid flat end wall 58 is attached to the mold shell 32 to close themold cavity. The core material is inserted into the mold cavity of themold shell 32 before the flat end wall 58 is attached across the upperopening of the mold shell 32. The initial fast spin of the mold causesthe epoxy under centrifugal force to migrate through the ceramic spheresto the perimeter of the mold, producing an epoxy-rich, solid layer atthe perimeter of the core 18. This effect eliminates the problem ofporosity of the core 18 since the rapid spin also causes the ceramicspheres to be flung outwardly from the center of the mold, increasingthe density of the ceramic spheres and closing any voids that may exist.Once these effects have been accomplished by the fast spin, the rotationspeed of the mold is reduced to prevent further migration of epoxy tothe perimeter, but slow speed rotation is maintained to prevent gravityfrom causing epoxy to run to the bottom as would happen in a stationarymold.

However, centrifuging the epoxy and the ceramic spheres to the perimeterof the mold creates a void between the ceramic spheres and the sleeve 22at the center of the mold. A mechanism which takes up this void includessilicone rubber bladders 56 mounted on the flat end walls 58, 60 of themold shell 32, as shown in FIG. 4. Because the flat end plates 58, 60close the mold shell 32 and prevent more core material from being added,the bladders 56 are expanded to take up the volume of material lost bythe compacting of the drum core material and give the cured core 18 anindented shape on both edges. After the mold shell 32 is closed, thebladders 56 are expanded inwardly by compressed air or another gastransported to the mold shell 32 by a hollow shaft 62 and a rotary aircoupling 64, as shown in FIG. 4. Air passageways 66 are formed in theflat end walls 58, 60 and communicate between the bladders 56 and thehollow shaft 62. The expanded bladders 56 compress the ceramic spheresinto a solid mass, and force the mass back into contact with the centersleeve 22 of the mold. The bladders 56 leave concave depressions on bothsides of the finished core 18, but these depressions only reduce themass of the cores 18, and insure that drum sections 12 fit togetherwithout interference when multiple drum sections 12 are mountedside-by-side on a common shaft. This is because the depressions insurethat the core material does not project beyond an ideal plane surfaceand cause interference between adjacent drum sections 12. The two sidesof the cores 18 are maintained symmetrical by using stiff rubberbladders 56 that require considerable air pressure to inflate. Thisstiffness ensures that the bladders 56 inflate equally, and since thewetted ceramic spheres offer little resistance to the bladder 56, thesides of the cores 18 become symmetrical.

LIQUID MOLD SYSTEM WITH PREFABRICATED CORE

After the core 18 is fabricated, the surface layer 20 is formed on thecore 18 using the liquid mold system in a separate mold apparatus 30 asshown in FIG. 5. A prefabricated solid core 18 is inserted into the moldshell 32, before the mold layer 54 is formed, by removing the retainingring 40 and placing the core 18 over the shaft 34 and into the moldshell 32 until it rests on the lower gasket 44. It is preferred that themold shell 32 have an inside diameter approximately 6 mm (0.24 inches)larger than the desired diameter of the finished drum section 12, andthe outer diameter of the core 18 be a few millimeters smaller than thediameter of the inner surface of the layer 54. The surface of the core18 can be toughened by machining or grinding to enhance adhesion of theepoxy. Adhesion promotion is especially important for drums that willoperate at high rotational speeds.

Next, the retaining ring 40 and upper gasket 42 are replaced and securedto the mold shell 32, so that the gasket 42 seals against the drumsection 12, and the gasket 44 is compressed by the core 18. The bottomof the drum section is sealed by the gasket 44 at the bottom of the moldshell 32 all of the way around the drum section 12 perimeter. After thecore 18 and retaining ring 40 are in place, rotation of the mold shell32 begins. The rotational speed is initially relatively low, suitablefor the initial steps of casting. This prevents deposition of thecasting resin from unbalancing the apparatus 30, disrupting theinterface between the mold liquid and the casting resin, and reducingthe quality of drum section surface 10. A starting speed of 450 RPM hasbeen found suitable.

Referring now to FIG. 6 as well as FIG. 5, when the desired speed isreached, the mold liquid which forms layer 54 is poured into the moldshell 32. This is accomplished by pouring the liquid into an annulargroove 68 at the inside diameter of the retaining ring 40. The liquidpasses through several small holes 70 spaced around the inside surfaceof the ring 40 into the space between the core 18 and the mold shell 32.The quantity of mold liquid that is used establishes a desired averageradius of the inner free surface from the center of rotation of the moldshell 32. This radius also becomes the outer radius of the surface layer20 when the drum section 12 is completed.

An activated resin is then poured into a second annular groove 72 in theretaining ring 40. The resin then flows through a separate set of smallholes 74 spaced around the inside surface of the ring 40 and located ona shorter radius than the first set of holes 70. The resin fills thespace between the free surface of the layer 54 and the core 18, and itcontacts and adheres to the surface of the core 18. An excess of boththe mold liquid and resin are poured into the retaining ring 40 grooves68, 72 so that the materials rise to levels having smaller radii thanthe corresponding layers, as shown in FIG. 6. The radii at which thelevels stabilize depends upon the ratio of the densities of the twoliquids. The excess liquids within the retaining ring grooves 68, 72serve as reservoirs which replenish liquids which are lost due toevaporation and to shrinkage of the resin as the resin cures during moldshell 32 rotation. The grooves 68, 72 and holes 70, 74 of the retainingring 40 are necessary for the insertion of liquids into the mold shell32 when a preformed core 18 is used as the core 18 blocks access to theouter diameter portion of the mold shell cavity.

The mold layer 54 and the surface substance 20 are poured into the moldshell 32 with the shaft 34 axis preferably oriented approximately 30°from horizontal, a 60° tilt. The mold shell 32 is preferably fixed inthis orientation to simplify the apparatus and obviate the need to pivotor tilt the mold shell 32. This mold shell 32 orientation, shown in FIG.6B, substantially eliminates any wedge shape, shown in FIG. 6A, which isotherwise created due to the force of gravity acting on and enlargingthe lower portion of the mold layer 54. Additionally, as the amount ofwedging is influenced by the rotational speed of the mold shell 32,higher rotational speeds further reduce wedging. While tilting the moldshell 32 to place the axis of the shaft 34 horizontal completelyeliminates the wedge shape, the resulting wedge using a 30° tilt is lessthan 0.025 mm (0.001 in), well within the limits which can be handled bya focus servo of the recording system.

A horizontal axis for the shaft 34 is not used as it presents airentrapment problems. Insertion of the liquids into the space between themold shell 32 and the core 18 must displace air so that air bubbles arenot trapped between the surface layer 20 and the core 18. If the moldshell 32 axis were horizontal, there would be no escape path for the airbubbles as the surface layer 20 material would move vertically with aflat surface to entrap air. Bubbles are not entrapped when the axis ofrotation of the mold shell 32 is not horizontal. The 30° tilt, combinedwith the initial 450 rpm rotational speed, creates the wedge shape inthe liquid mold layer 54 and provides a relative angle between the core18 and the free surface of the mold layer 54. Resin inserted into thespace between the mold layer 54 and the core 18 rises with uniformthickness to meet the core 18. It meets the core 18 first at the closedend of the mold shell 32, causing air between the resin and the core 18to move toward and through the small space between the core 18 and theretaining ring 40.

Once the volume of space between the mold shell 32 and the core 18 hasbeen filled, the mold shaft axis is returned to a vertical position,shown in FIG. 6A, if the mold shell 32 is pivotable, and any remainingwedge shape of layers 20 and 54 gives way to layers of uniform thicknessacross the width of the core. The rotational speed of the mold shell 32is now increased to increase the centrifugal force on both layers sothat the resin cures under high pressure in contact with the mold layer54 and the core 18. Shrinkage of the resin is compensated for by theaddition of resin from the reservoir contained in the retaining ring 40.

After the surface layer 20 has cured, rotation can be stopped. Due tothe high viscosity of the epoxy and because the entire volume betweenthe core 18 and the mold shell 32 is filled, wave motion caused byvibration is negligible. Any mold layer 54 liquid that spills out of themold shell 32 is caught in a shallow tray (not shown) located beneaththe mold shell 32. Once rotation has stopped, the retaining ring 40 isremoved, and the finished drum section 12 is removed from the mold.Since the mold surface is liquid, the drum section 12 will not stick toor damage the mold surface. Where beeswax is used, the drum section 12can be removed from the mold 32 end surface by heating it sufficientlyto melt the beeswax at the bottom and then turning the mold upside downand allowing the drum section 12 to slide down the shaft 34.

AS slight overfilling of the mold during casting assures completecoverage of the core 18 by the surface layer 20, some excess materialmay be present at the edges of the finished part. This material can beremoved by machining, as long as damage to the curved peripheral surface10 is prevented. Once final edge finishing of the drum section 12 iscomplete, the recording material can be applied to the surface 10 of thesurface layer 20. An overcoat is applied, and edge finishing isperformed as described above.

SOLID MOLD SYSTEM WITH PREFABRICATED CORE

Alternatively, when forming a surface layer 20 for a drum section 12,the mold layer 54 can be formed using a hardenable casting epoxy orprepolymer composition, called the mold resin. Although this method isnot as simple as using a liquid mold system and requires more steps, itrepresents an improvement over known prior art systems. In this system,the liquid mold resin is deposited into a spinning mold shell 32 and isallowed to cure while maintaining the rotational speed, thereby forminga solid tapered cylindrical mold layer 54 having a shape described byequation 4.

Commercially available casting epoxies which have a very low shrinklevel during curing and do not contain volatile solvents or producereaction products which bubble or form other surface defects aresuitable mold resin materials for layer 54. Epoxies adhere well to cleanmetals and other durable materials, even without adhesion promotingprimers, and are curable at temperatures near room temperature.Additionally, some epoxies provide a very brittle, optically polishablesurface. Because of the need for high surface smoothness, the liquidmaterials used to form the mold and the drum surface layer 20 should notcontain particulate impurities of any significant size. This can beachieved by filtering using filters having pore sizes in the submicronsize range. Liquid materials having this level of purity aremicroelectronic grade materials. A particularly suitable mold resin is atwo-part liquid casting epoxy prepared by blending 100 parts by weightof a low viscosity bisphenol a compound with 55 parts by weight of amodified cycloaliphatic amine hardener. The mixed composition isaspirated in a vacuum chamber to remove any air bubbles entrained duringmixing, as is common when casting polymeric materials.

The aspirated resin blend is deposited onto the rotating inner wall ofthe mold shell 32. The speed of the mold shell 32 is held constant atapproximately 2500 rpm until the mold resin forming the layer 54hardens. While the free surface of mold layer 54 will be smooth aftercuring, it is not yet suitable for use as a mold. After stoppingrotation, any protrusions caused by particulate impurities whichtraveled to the inner surface of the mold layer 54 must be removed bypolishing. Also, as many casting resins adhere to, chemically reactwith, or diffuse into the mold layer 54, the mold layer 54 must beprovided with a protective coating. The coating isolates the mold layer54 from the materials used in casting but does not reduce its smoothnessor adversely affect its concentricity or other dimensions. The coating,such as vacuum deposited chromium, also effects a release of the outersurface 20 from the solid mold layer 54 by preventing adhesion of theouter surface 20 to the mold layer 54. Also, wiping a very thin layer ofrelease agent on the mold layer 54 before inserting the surface layermaterial further prevents adhesion.

The mold resin forms a coating having a thickness causing its surface tobe equidistant from the axis of rotation of the mold shell 32 to insureconcentricity, flatness, and roundness of the surface. Due to gravity,the lower portion of the surface of mold layer 54 is slightly thicker sothat the surface forms a wedge or conical shape. This shape assists therelease of the finished drum section 12 from the mold shell 32. A wedgeof any taper can be obtained by controlling the rotation speed of themold shell 32. A difference in the upper and lower radii ofapproximately 0.1 mm (0.004 in) is acceptable.

When the drum section 12 is to be formed without a prefabricated core18, fabrication proceeds as described above.

Where a preformed core 18 is used, the retaining ring 40 is removed andthe core 18 is placed over the shaft 34 and into the mold, until itrests on the lower gasket 44. The retaining ring 40 and upper gasket 42are then replaced and secured to the mold shell 32, and the bottom ofthe drum section 12 is sealed at the bottom of the mold shell 32 all theway around its perimeter as done when using a liquid mold system. Next,the surface layer 20 is formed by filling the gap surrounding the core18 with casting resin 76.

The casting resin 76 can be injected into the sealed gap between themold layer 54 and the core 18, using top and bottom openings provided inthe gap, as shown in FIG. 7. The top opening is connected to a vacuumsource 78 by tube 80 to remove air from the gap. Alternatively, theentire mold may be placed in a vacuum chamber, provided that castingresin 76 remains exposed to the atmosphere, or is pressurized. Thebottom opening is connected to a valved tube 82, which is immersed incasting resin 76 in a container 84. The casting resin 76 is allowed toflow into the gap until it begins to exit from the top opening,whereupon the openings are closed, and the casting resin 76 in the gapis allowed to cure.

Once the casting resin 76 has cured, the retaining ring 40 is removed,and the core 18 with a cast surface layer 20 around its outer peripheryis removed from the mold. The brittle resin in the inlet and outlettubes 80, 82 is easily broken off during removal of the finished drumsection 12. Removal is simplified if the drum section 12 is providedwith grasping means to pull it out, or if a set of ejector pins (notshown) pushes out the drum section 12 from the mold shell 32. The lowergasket 44 can be compressed during the sealing of the retaining ring 40,and the release of this compression may be sufficient to release thedrum section 12 from the mold. Additionally, the taper or wedging in themold layer 54, which can be controlled by controlling the centrifugalforce according to the equations discussed above, increases the physicalclearance between the mold layer 54 and the drum section 12 as the partsseparate to insure that neither surface is damaged. Thus, in thissituation, wedging is beneficial. The drum section 12 is then slid fromthe shaft 34 by turning the mold upside down. These drum sections 12 mayrequire slight edge finishing, especially at the entrance and exitholes. Finishing can be accomplished by conventional machining, asdescribed above. Finally, a magneto-optic recording layer can bedeposited onto the surface of the drum section 12. A protective overcoatlayer may also be applied using the same molding techniques.

Numerous characteristics, advantages, and embodiments of the inventionhave been described in detail in the foregoing description withreference to the accompanying drawings. However, the disclosure isillustrative only and the invention is not intended to be limited to theprecise embodiments illustrated. Various changes and modifications maybe effected therein by one skilled in the art without departing from thescope or spirit of the invention. For example, differently-sized molds,operating at different speeds may be used to form smaller or larger drumsections.

We claim:
 1. A cylindrical rotatable optical recording drum for use inerasable optical recording comprising:an outer cylindrical surface onwhich reading and writing can be performed; and a core including aninner core layer comprising hollow spheres mixed with a resin, whereinthe radially outer surface of the inner core layer has a higher ratio ofresin to spheres than the radially inner surface of the inner corelayer.
 2. A cylindrical rotatable optical recording drum for use inerasable optical recording comprising:an outer cylindrical surface onwhich reading and writing can be performed; and a core including aninner core layer comprising hollow spheres mixed with a resin and anouter core layer disposed between the inner core layer and the outercylindrical surface, wherein the outer core layer comprises hollowspheres mixed with a resin, and wherein the radially outer surface ofthe outer core layer has a higher ratio of resin to spheres than theradially inner surface of the outer core layer.